Summary of Work: Rare replication errors are corrected by post-replication DNA mismatch repair (MMR), and loss of MMR increases mutation rates. The goals of this project are to understand the biochemistry and genetics of MMR in normal eukaryotic cells, and how mutations in MMR genes lead to environmentally associated human diseases. This year we had four accomplishments. 1) We demonstrated that Msh6 and Msh3 contain consensus PCNA binding motifs and bind to PCNA. We provided evidence that this interaction is functionally important for MMR. The results suggest that the MMR complex may be physically associated with the replication fork, which has implications for coordination between mismatched DNA binding by Msh2-Msh6/3 and recognition of the strand discrimination signal. 2) We demonstrated that over expression of Mlh1 or Pms1 in wild-type yeast cells yields a dominant mutator effect that is due to suppression of MMR activity, and we provided evidence for partially defective mismatch repair in bladder cancer cells. Both results suggest that cancer could be initiated by imbalanced expression of MMR genes leading to reduced MMR and elevated mutation rates. 3) We demonstrated that hMlh1 interacts with the BLM helicase, the protein that is defective in Blooms Syndrome. The functional relevance of this interaction remains to be determined, since BLM defective cells are proficient in MMR. 4) We successfully developed a new strategy to purify the Mlh1-Pms1 heterodimer in amounts sufficient for biochemical and biophysical analysis that are now underway. Studies of MMR are important for understanding the mechanism of MMR, the risk posed to individuals in the population by exposure to DNA damaging agents and the molecular basis for the initiating events in cancer and its subsequent treatment.